Regardless of who pays for it, how high it is, how long, or how beautiful, the Trump Wall will be useless for keeping out immigrants trying to cross the border illegally.

NOGALES, AZ - MAY 14: A steel wall separates Nogales, Sonora, Mexico (L) from a U.S. Border Patrol agent in his sector May 14, 2006 in Nogales, Arizona. U.S. President George W. Bush will address the nation on the immigration issue May 15. (Photo by Jeff Topping/Getty Images)

During their third and final debate on Wednesday, Oct. 19, Donald Trump and Hillary Clinton will discuss immigration, among other topics. At a rally in Florida on Oct. 12, Trump repeated his promise that he would build a wall on the southern border and that Mexico will pay for it “100 percent.… They just don’t know it yet.” Regardless of who pays for it, how high it is, how long, or how beautiful, the Trump Wall will be useless for keeping out immigrants trying to cross the border illegally.

Why? Because the huge numbers of people that Trump intends to stop at the border won’t exist, a result of crashing marriage and birthrates in the rest of the hemisphere. Those “missing” people are already showing up in border-crossing data, which show that “net zero” is the new normal. Other reasons to secure the southern border remain — keeping out criminals and terrorists, for instance — but repelling huge waves of unauthorized immigrants is no longer one of them.

Fifty years ago, the average Mexican woman gave birth to seven children during her lifetime, according to the World Bank. Today, the number is two. Beginning in the 1940s and peaking in the 1960s, Mexico’s huge growth ratequadrupled the country’s population between 1950 and 2010. Now, it’s back to the birthrate it had in 1895. Mexico is not alone. Birthrates and population growth have crashed in Latin America and the Caribbean. The average number of births per woman in the region has dropped from six in 1960 to just over two in 2014. (Even the outliers — Bolivia, Haiti, and Guatemala — are now just half the 1960 rate.) Everywhere else, the demographic numbers are starting to look very much like those up north.

Does this sound familiar? The economic, cultural, and social trends that have driven down birthrates in the United States over the last 50 years have now been fully assimilated in Latin America. Fewer people are getting married, and more are getting divorced. Couples are having less children and having them later. In Mexico, there has been a 32 percent drop since 1980 in the marriage rate and a 345 percent increase in the divorce rate. The trends in the rest of Latin America have been the same, though not as dramatic. (Although civil unions have in some cases replaced marriages, these arrangements tend to have significantly fewer children.)

Fewer people means fewer workers. Labor shortages — once unheard of — have already cropped up on both sides of the U.S. southern border. Despite wage rates of $14-17 per hour, farmers in Washington state and California last year were forced to let crops rot because there were simply not enough people to harvest them — Mexican-born or otherwise. The same shortage has affected the U.S. construction industry, which has lost almost 600,000 Mexican-born workers since 2007. Meanwhile, auto plants in Mexico are also having trouble attracting and keeping workers, despite big pay increases. If and when the U.S. economy returns to healthy growth rates, we may see sharp spikes again in illegal immigration but at much lower absolute numbers compared with the 1990s. For example, although the wage gap between the United States and Latin America remains huge, it is failing to draw enough workers from a labor pool that is shrinking by the day.

The steep drop in Mexico’s population growth from 1980 to 1990 started showing up in fewer unauthorized immigrants, beginning around 2004. This makes sense since people are most likely to migrate between the ages of 20 and 30. According to Pew Research Center data, between 1995 and 2000, more than 2.9 million Mexicans immigrated to the United States. Between 2005 and 2010, that number had more than halved to less than 1.4 million — fewer than the number of Mexican immigrants who had returned to Mexico. Since 2010, the incoming rate has held steady, but in 2015 the number of Mexican immigrants caught at the border was the lowest in 50 years.

There are still plenty of good reasons to secure the southern border. Keeping out terrorists, drug smugglers, and sex traffickers to name a few. But to prevent a never-ending torrent of unauthorized immigrants? Those days are gone. In fact, in recent years an immigrant who is residing in the country illegally is more likely to be from Asia, Central America, or Africa — arriving on a plane and overstaying a legal tourist visa. Trump’s Wall may be high, but it won’t be high enough to keep out passenger jets.

Photo credit: Jeff Topping/Getty Images

Richard G. Miles is the director of the U.S.-Mexico Futures Initiative at the Center for Strategic and International Studies. From 2007 to 2008, he handled Mexican affairs on the U.S. National Security Council staff.